Since we started indexing the CRLF-adjusted size of messages,
we can take an order-of-magnitude speedup for certain MUAs
which fetch this attribute without needing much else.
Admins are encouraged to --reindex existing inboxes for IMAP
support, anyways. It won't be fatal if it's not reindexed, but
some client bugs and warnings can be fixed and they'll be able
to support more of IMAP.